INTRODUCTION: Pain is the commonest reason that patients present to an emergency department (ED), but it is often not treated effectively. Patient controlled analgesia (PCA) is used in other hospital settings but there is little evidence to support its use in emergency patients. We describe two randomised trials aiming to compare PCA to nurse titrated analgesia (routine care) in adult patients who present to the ED requiring intravenous opioid analgesia for the treatment of moderate to severe pain and are subsequently admitted to hospital. METHODS AND ANALYSIS: Two prospective multi-centre open-label randomised trials of PCA versus routine care in emergency department patients who require intravenous opioid analgesia followed by admission to hospital; one trial involving patients with traumatic musculoskeletal injuries and the second involving patients with non-traumatic abdominal pain. In each trial, 200 participants will be randomised to receive either routine care or PCA, and followed for the first 12 h of their hospital stay. The primary outcome measure is hourly pain score recorded by the participant using a visual analogue scale (VAS) over the 12 h study period, with the primary statistical analyses based on the area under the curve of these pain scores. Secondary outcomes include total opioid use, side effects, time spent asleep, patient satisfaction, length of hospital stay and incremental cost effectiveness ratio. ETHICS AND DISSEMINATION: The study is approved by the South Central-Southampton A Research Ethics Committee (REC reference 11/SC/0151). Data collection will be completed by August 2013, with statistical analyses starting after all final data queries are resolved. Dissemination plans include presentations at local, national and international scientific meetings held by relevant Colleges and societies. Publications should be ready for submission during 2014. A lay summary of the results will be available to study participants on request, and disseminated via a publically accessible website. REGISTRATION DETAILS: The study is registered with the European Clinical Trials Database (EudraCT Number: 2011-000194-31) and is on the ISCRTN register (ISRCTN25343280).